Fabrication of Si heterojunction solar cells using P-doped Si nanocrystals embedded in SiN x films as emitters
© Wu et al.; licensee Springer. 2013
Received: 2 June 2013
Accepted: 26 October 2013
Published: 5 November 2013
Si heterojunction solar cells were fabricated on p-type single-crystal Si (sc-Si) substrates using phosphorus-doped Si nanocrystals (Si-NCs) embedded in SiN x (Si-NCs/SiN x ) films as emitters. The Si-NCs were formed by post-annealing of silicon-rich silicon nitride films deposited by electron cyclotron resonance chemical vapor deposition. We investigate the influence of the N/Si ratio in the Si-NCs/SiN x films on their electrical and optical properties, as well as the photovoltaic properties of the fabricated heterojunction devices. Increasing the nitrogen content enhances the optical gap E04 while deteriorating the electrical conductivity of the Si-NCs/SiN x film, leading to an increased short-circuit current density and a decreased fill factor of the heterojunction device. These trends could be interpreted by a bi-phase model which describes the Si-NCs/SiN x film as a mixture of a high-transparency SiN x phase and a low-resistivity Si-NC phase. A preliminary efficiency of 8.6% is achieved for the Si-NCs/sc-Si heterojunction solar cell.
Materials consisting of silicon nanocrystals (Si-NCs) embedded in a dielectric matrix are one promising candidate to realize Si-based third-generation photovoltaic devices owing to their potential benefits of utilizing the visible light of terrestrial solar spectrum and overcoming the efficiency limit of crystalline Si (c-Si) solar cells [1–5]. Sub-stoichiometric Si-based dielectric materials, such as SiO x , SiN x , and SiC x , have been investigated for synthesis of Si-NCs [6–11]. The formation of Si-NCs is based on phase segregation and crystallization in Si-rich dielectric films during the post-annealing process .
The low conductivity of Si-NCs embedded in dielectric films limits their applications for the manufacturing of optoelectronic devices. For this reason, impurity doping in Si-NCs embedded in SiO2 has been demonstrated to modify the electrical properties of the layers, although there is some debate about the feasibility of doping in Si-NCs [13, 14]. In addition to impurity doping, the choice of the surrounding dielectric matrix also plays a crucial role in charge carrier transport. Although the formation of Si-NCs in the SiO2 matrix has been investigated in detail [12, 15], the carrier transport ability in the Si-NC network is generally insufficient due to the large energy barrier of the surrounding oxide matrix. Charge carrier transport through narrower bandgap dielectrics, such as Si3N4 or SiC, seems to be more feasible. Compared with SiO2 and SiC, Si3N4 may offer a compromise as a dielectric matrix for the Si-NC network used in solar cell applications since it possesses a medium bandgap (approximately 5.3 eV) which could reduce the energy barrier for carrier transport and also effectively avoid parasitic absorption. However, doped Si-NCs embedded in a SiN x matrix (Si-NCs/SiN x ) have not received much attention.
In this work, we present initial fabrication and characterization results of Si heterojunction solar cells using P-doped Si-NCs/SiN x films as emitters. The P-doped Si-NCs/SiN x films were prepared by electron cyclotron resonance chemical vapor deposition (ECRCVD) followed by high-temperature annealing, and the influence of the chemical composition (N/Si ratio) on their physical properties was investigated. The photovoltaic properties of the fabricated heterojunction devices were also examined as a function of the N/Si composition ratio in the P-doped Si-NCs/SiN x films.
Fifty-nanometer-thick, homogeneous Si-rich silicon nitride (SRN) films containing phosphorus were deposited by a homemade ECRCVD system on single-side polished p-type (100) single crystalline Si (sc-Si) substrates with a thickness of 675 μm and a resistivity in the range of 5 to 20 Ω cm. Before placing into the deposition chamber, Si substrates were cleaned with acetone and rinsed in deionized water followed by removal of native oxide on Si wafers using a diluted HF dip (5%). The mixed SiH4, N2, Ar, and PH3 gases were then introduced into the deposition chamber at 10 mTorr for film growth. The applied microwave power and the substrate temperature were kept on 700 W and 200°C, respectively. In order to study the influence of the Si/N ratio on film properties, both SiH4 and PH3 flow rates were kept constant during film growth, while the gas mix ratio (Rc) defined as N2/SiH4 was varied in the range 0.73 ≤ Rc ≤ 0.83. The formation of Si-NCs in as-deposited SRN films was carried out by post-growth annealing in a quartz tube furnace at 950°C in N2 ambient. Samples with a 1 cm × 1 cm area were used for subsequent fabrication of heterojunction solar cells. Aluminum films deposited by electron gun evaporation were used as contact electrode layers in the solar cells. The front contact on top of the Si-NCs/SiN x film was defined by a shadow mask during Al deposition, while the rear contact covered the full back area of the cell. After metallization, the samples were heated at 500°C for 3 min to improve the electrical properties of the contacts.
For the characterization, the bonding configurations of the Si-NCs/SiN x films were identified by X-ray photoelectron spectroscopy (XPS). Micro-Raman spectroscopy and transmission electron microscopy (TEM) were used to investigate the crystallization behavior in SRN films after post-growth annealing. Fused quartz wafers were used as substrates for Raman studies to avoid the signal contribution from Si substrates during Raman measurements. X-ray diffraction (XRD) was used to evaluate the Si-NC size of annealed samples. The photovoltaic properties of the fabricated heterojunction solar cells were evaluated at room temperature based on the illuminated current density versus voltage (J-V) and the internal quantum efficiency (IQE) characteristics under 1-sun air mass 1.5 global spectrum.
Results and discussion
From Figure 6, the Jsc is increased from 21.3 to 28.2 mA/cm2 with increasing Rc value. This trend could be ascribed to the lower parasitic absorption in the Si-NCs/SiN x film with a higher Rc value since the increasing Si-NC phase could result in a reduction in the optical gap of the film due to its higher absorption coefficient, as mentioned above (see Figure 4b). To better understand the difference in Jsc among the heterojunction solar cells with various Rc values, losses of the Jsc in the devices were investigated from their IQE data by spectral response measurements. As shown in Figure 5b, the heterojunction device with a higher Rc Si-NCs/SiN x film shows a higher IQE in the short wavelength regime, especially for the wavelength range between 400 and 700 nm, while the IQE spectra in the infrared wavelength regime (>900 nm) are similar for all heterojunction solar cells, implying that recombination of photo-generated charge carriers in the absorber layer is almost the same in all heterojunction devices . Moreover, as depicted in Figure 4a, the obvious variations in the absorption spectra of the P-doped Si-NCs/sc-Si films with various Rc values could be observed at photon energies above 1.8 eV (approximately <700 nm), which shows good correspondence with the trends in the IQE data. Therefore, it is speculated that the difference in Jsc losses among the devices could be attributed to the parasitic absorption in the emitter layer. More photons in the visible spectrum would be absorbed with increasing volume fraction of the Si-NCs in the P-doped Si-NCs/sc-Si film, leading to the limitation in the available solar spectrum in the device, as well as the degradation of the Jsc.
In this report, we have investigated the feasibility of using P-doped Si-NCs/SiN x films as emitters on p-type sc-Si substrates for fabrication of Si-based heterojunction solar cells. From XPS analysis of the P-doped Si-NCs/SiN x films, the P 2p signal only attributed to Si-P or P-P bonds indicates that the P atoms may exist inside Si-NCs. The electrical and optical properties of the P-doped Si-NCs/SiN x material are strongly influenced by its chemical composition (N/Si ratio). The optical gap E04 is enhanced with increasing nitrogen content, while the conductivity is deteriorated. These trends could be interpreted by a bi-phase model, where the SiN x phase contributes to the optical gap enhancement and the Si-NC phase promotes charge carrier transport. Therefore, the Jsc is increased with increasing N/Si ratio in the Si-NCs/SiN x layer, while the FF is reduced. The best cell parameters obtained are Voc of 500 mV, Jsc of 28.2 mA/cm2, FF of 65.2%, and conversion efficiency of 8.6% from the heterojunction solar cell with a Rc = 0.79 Si-NCs/SiN x emitter. Further device optimization is required to improve the photovoltaic efficiency.
This research was supported by the National Science Council of Taiwan under grant nos. 101-2221-E-008-041 and 101-2622-E-008-015-CC3.
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